Field of the Invention
This invention relates to a process for the separation of non-ferrous metals from various mineral sources amenable to forming soluble salts of such metals when treated under conditions as hereinafter specified. More particularly, this invention relates to a process for the recovery of substantially quantitative amounts of non-ferrous metals, and especially precious or noble metals, from various mineral sources.
The recovery of non-ferrous metals from metal bearing ores has been practiced for centuries with varying degrees of success. It is well known that metals may occur in nature in their elemental state, as salts, complexes, oxides, sulfides and in various other forms. Because of the complexity of mineral recovery, the field of metallurgy has become a highly sophisticated science.
In general, presently used methods for metal recovery from raw ores usually consist of the steps of crushing, grinding, washing, leaching or smelting followed by recovery such as gravity separation, and electrolytic or chemical precipitation.
Some ores or concentrates may be treated in solution using a leach solution or hot molten materials to dissolve the minerals at some stage in their treatment. From the solutions the metals are separated by various means such as selective precipitation, cementation, electrowinning and gravity. The solution can be an aqueous salt, acid or base or a molten fluid such as is used in smelting.
Large operations for the recovery of non-ferrous metal sulfides such as copper or zinc usually consist of mining the raw material, crushing it, further reducing the particle size by grinding, floatation to concentrate the ore, smelting and electrolytic refining. Metal oxides are usually mined, crushed, leached with an acid solution and electrowon or otherwise precipitated from solution.
Precious metals may be in elemental form and separated by placer operations, i.e., gravity separation by washing through sluice boxes and separated on finishing tables. On the other hand, precious metals such as gold and silver may be treated by crushing the ore, fine grinding it and complexing the precious metal with a cyanide to form a solution and recovering the precious metal.
The noble or precious metals such as gold, silver and the platinum group, (i.e., platinum, palladium, rhodium, ruthenium, iridium and osmium) usually occur in fractions of an ounce per ton of ore. As stated, these metals can occur in their elemental state but are also known to be alloyed with iron, copper, nickel and other metals as well as being combined, complexed or otherwise chemically or physically bound up in many other ways. Gold, for example, is classified as occurring in a free elemental state or as refractory, float gold, telluride, rusty gold, coated, and in several other forms. The coating can consist of hydrocarbons, iron oxides, manganese, tellurium or an encasement in quartz or other minerals. Normally it has been thought that the finer the grinding the better will be the liberation of minerals in a solution or floatation process. However, when the particles are fine enough to float on water they become difficult to recover and are easily lost.
Recovery of metals from ores usually is considered good if ninety-five percent of the assayed quantity is recovered. Many operations yield only from 70 to 90 percent. All recoveries are based on assay analysis of samples taken at various stages of processing such as heads, concentrates, tailings, slag, crude and refined metals. Each ore is different. Some are very complex containing many metals in varying amounts and combined into various compounds or mineral forms.
Assay reports are based on the assumption that the techniques used to dissolve or extract the metal from the sample have been complete and exhaustive. In reality the assay reported is only as effective as the ability of the procedure used to identify all of the metal present in the sample. There are so called unassayable ores wherein the minerals are combined in such a way that they cannot be analyzed by conventional techniques. It has also been found in most cases that insofar as precious or noble metals are concerned, present assay methods only identify a portion of the metal present in the sample. It is obvious that if this observation is correct present methods of recovery of precious metals are inadequate and vast quantities of gold, silver and the platinum group are being dumped as waste materials from various metal recovery operations.